1. Field of the Invention
The present invention relates to a plasma processing method of subjecting a target object, such as a semiconductor wafer or an LCD (liquid crystal display) substrate, to a plasma process, such as an etching process or the like.
2. Description of the Related Art
Etching apparatuses utilizing plasma are conventionally employed for etching an insulating film, arranged on, e.g., a semiconductor wafer, to form a contact hole, when a semiconductor device is manufactured. Among the etching apparatuses, the parallel plate type which has upper and lower electrodes facing each other in a process chamber is commonly used, since this type is suitable for processing a wafer having a relatively large diameter.
In an etching apparatus of the parallel plate type, since a wafer should be held in a place during an etching process, an electrostatic chuck is arranged on the lower electrode, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 4-51542. The electrostatic chuck has a thin electrode covered with an insulating film. A DC (direct current) voltage is applied to the thin electrode, so that a wafer is attracted and held by an electrostatic force, i.e., a Coulomb's force, generated by the voltage.
With an increase in the number of repeated operations of holding wafers to be processed, electric charge is apt to remain on the surface of the insulating film of the electrostatic chuck. The residual electric charge has a polarity opposite to the DC voltage applied for attracting and holding the wafers. As a result, a next wafer may not be attracted and held even if a DC voltage is applied to the electrostatic chuck. The following methods have been developed in order to solve this problem.
Jpn. Pat. Appln. KOKOKU Publication No. 63-36138 discloses a method of supplying a gas into a process chamber at an electric discharge pressure and causing RF (radio frequency) discharge, using electrodes in the process chamber, after a wafer is separated from the chuck, in order to prevent malfunction of the chuck in its attracting and holding function. This method is referred to as an electric charge removing method using plasma. Above mentioned Jpn. Pat. Appln. KOKAI Publication No. 4-51542 discloses a method of supplying an inactive gas and causing RF discharge after a wafer is separated from an electrostatic chuck.
In both of the conventional methods described above, residual electric charge is removed under a discharge condition the same as that of the main etching step. As a result, problems occur such that a reaction product is deposited on the surface of an electrostatic chuck (so called adhesion of deposition), and the surface of the electrostatic chuck is subjected to spattering.
In addition, in the conventional methods, plasma discharge is caused for removing residual electric charge after an etched wafer is transferred out of a process chamber, and then a next wafer is transferred into the process chamber after the removal of the residual electric charge is completed. In other words, the steps of transferring a wafer out of and into the process chamber cannot be performed at the same time as the step of removing electric charge, thereby lowering throughput.
Where a DC voltage is applied to an electrostatic chuck before a wafer is mounted on the electrostatic chuck, another problem is caused. To be specific, an etching apparatus of this kind has support means, such as lifter pins arranged in a work table and having distal ends projectable from an electrostatic chuck. The support means is used for putting a wafer, which has been transferred into a process chamber by transfer means, such as a transfer arm, onto the electrostatic chuck, and for separating the processed wafer from the electrostatic chuck and putting it onto the transfer means.
Since the support means is grounded through a resistor, DC (direct current) discharge is caused between the support means and the electrostatic chuck when the support means is lifted, if a DC voltage is applied to the electrostatic chuck before a wafer is mounted on the electrostatic chuck. For example, where a plus DC voltage is applied to the electrostatic chuck, the chuck is charged with negative electric charge due to the DC discharge, so that its attracting and holding function may be lowered.